There is a need to identify new sources of chemical energy and methods for its conversion into alternative transportation fuels, driven by many concerns including environmental, health, safety issues, and the inevitable future scarcity of petroleum-based fuel supplies. The number of internal combustion engine fueled vehicles worldwide continues to grow, particularly in the midrange of developing countries. The worldwide vehicle population outside the U.S., which mainly uses diesel fuel, is growing faster than inside the U.S. This situation may change as more fuel-efficient vehicles, using hybrid and/or diesel engine technologies, are introduced to reduce both fuel consumption and overall emissions. Since the resources for the production of petroleum-based fuels are being depleted, dependency on petroleum will become a major problem unless non-petroleum alternative fuels, in particular clean-burning synthetic diesel fuels, are developed. Moreover, normal combustion of petroleum-based fuels in conventional engines can cause serious environmental pollution unless strict methods of exhaust emission control are used. A clean burning synthetic diesel fuel can help reduce the emissions from diesel engines.
The production of clean-burning transportation fuels requires either the reformulation of existing petroleum-based fuels or the discovery of new methods for power production or fuel synthesis from unused materials. There are many sources available, derived from either renewable organic or waste carbonaceous materials. Utilizing carbonaceous waste to produce synthetic fuels is an economically viable method since the input feed stock is already considered of little value, discarded as waste, and disposal is often polluting. Alternatively, one can use coal as a feedstock to upgrade low grade dirty solid fuel to a value added convenient clean liquid fuel, such as high quality, environment friendly synthetic diesel or other hydrocarbon fuels.
Liquid transportation fuels have inherent advantages over gaseous fuels, having higher energy densities than gaseous fuels at the same pressure and temperature. Liquid fuels can be stored at atmospheric or low pressures whereas to achieve liquid fuel energy densities, a gaseous fuel would have to be stored in a tank on a vehicle at high pressures that can be a safety concern in the case of leaks or sudden rupture. The distribution of liquid fuels is much easier than gaseous fuels, using simple pumps and pipelines. The liquid fueling infrastructure of the existing transportation sector ensures easy integration into the existing market of any production of clean-burning synthetic liquid transportation fuels.
The availability of clean-burning liquid transportation fuels is a national priority. Producing synthesis gas (a mixture of hydrogen and carbon monoxide, also referred to as synthesis gas) cleanly and efficiently from carbonaceous sources, that can be subjected to a Fischer-Tropsch process to produce clean and valuable synthetic gasoline and diesel fuels, will benefit both the transportation sector and the health of society. Such a process allows for the application of current state-of-art engine exhaust after-treatment methods for NOx reduction, removal of toxic particulates present in diesel engine exhaust, and the reduction of normal combustion product pollutants, currently accomplished by catalysts that are poisoned quickly by any sulfur present, as is the case in ordinary stocks of petroleum derived diesel fuel, reducing the catalyst efficiency. Typically, Fischer-Tropsch liquid fuels, produced from synthesis gas, are sulfur-free, aromatic free, and in the case of synthetic diesel fuel have an ultrahigh cetane value.
Biomass material is the most commonly processed carbonaceous waste feed stock used to produce renewable fuels. Waste plastic, rubber, manure, crop residues, forestry, tree and grass cuttings and biosolids from waste water (sewage) treatment are also candidate feed stocks for conversion processes. Biomass feed stocks can be converted to produce electricity, heat, valuable chemicals or fuels. California tops the nation in the use and development of several biomass utilization technologies. Each year in California, more than 45 million tons of municipal solid waste is discarded for treatment by waste management facilities. Approximately half this waste ends up in landfills. For example, in just the Riverside County, California area, it is estimated that about 4000 tons of waste wood are disposed of per day. According to other estimates, over 100,000 tons of biomass per day are dumped into landfills in the Riverside County collection area. This municipal waste comprises about 30% waste paper or cardboard, 40% organic (green and food) waste, and 30% combinations of wood, paper, plastic and metal waste. The carbonaceous components of this waste material have chemical energy that could be used to reduce the need for other energy sources if it can be converted into a clean-burning fuel. These waste sources of carbonaceous material are not the only sources available. While many existing carbonaceous waste materials, such as paper, can be sorted, reused and recycled, for other materials, the waste producer would not need to pay a tipping fee, if the waste were to be delivered directly to a conversion facility. A tipping fee, presently at $30-$35 per ton, is usually charged by the waste management agency to offset disposal costs. Consequently not only can disposal costs be reduced by transporting the waste to a waste-to-synthetic fuels processing plant, but additional waste would be made available because of the lowered cost of disposal.
The burning of wood in a wood stove is a simple example of using biomass to produce heat energy. Unfortunately, open burning of biomass waste to obtain energy and heat is not a clean and efficient method to utilize the calorific value. Today, many new ways of utilizing carbonaceous waste are being discovered. For example, one way is to produce synthetic liquid transportation fuels, and another way is to produce energetic gas for conversion into electricity.
Using fuels from renewable biomass sources can actually decrease the net accumulation of greenhouse gases, such as carbon dioxide, while providing clean, efficient energy for transportation. One of the principal benefits of co-production of synthetic liquid fuels from biomass sources is that it can provide a storable transportation fuel while reducing the effects of greenhouse gases contributing to global warming. In the future, these co-production processes could provide clean-burning fuels for a renewable fuel economy that could be sustained continuously.
A number of processes exist to convert coal and other carbonaceous materials to clean-burning transportation fuels, but they tend to be too expensive to compete on the market with petroleum-based fuels, or they produce volatile fuels, such as methanol and ethanol that have vapor pressure values too high for use in high pollution areas, such as the Southern California air-basin, without legislative exemption from clean air regulations. An example of the latter process is the Hynol Methanol Process, which uses hydro-gasification and steam reformer reactors to synthesize methanol using a co-feed of solid carbonaceous materials and natural gas, and which has a demonstrated carbon conversion efficiency of >85% in bench-scale demonstrations.
More recently, a process was developed in our laboratories to produce synthesis gas in which a slurry of particles of carbonaceous material in water, and hydrogen from an internal source, are fed into a hydro-gasification reactor under conditions to generate rich producer gas. This is fed along with steam into a steam pyrolytic reformer under conditions to generate synthesis gas. This process is described in detail in Norbeck et al. U.S. patent application Ser. No. 10/503,435 (published as US 2005/0256212), entitled: “Production Of Synthetic Transportation Fuels From Carbonaceous Material Using Self-Sustained Hydro-Gasification.”
In a further version of the process, of particular interest here, using a steam hydro-gasification reactor (SHR) the carbonaceous material is heated simultaneously in the presence of both hydrogen and steam to undergo steam pyrolysis and hydro-gasification in a single step. This process is described in detail in Norbeck et al. U.S. patent application Ser. No. 10/911,348 (published as US 2005/0032920), entitled: “Steam Pyrolysis As A Process to Enhance The Hydro-Gasification of Carbonaceous Material.” The disclosures of U.S. patent application Ser. Nos. 10/503,435 and 10/911,348 are incorporated herein by reference.
Producing synthesis gas via gasification and producing a liquid fuel from synthesis gas are totally different processes. Synthesis gas is produced using a steam methane reformer (SMR), a reactor that is widely used to produce synthesis gas for the production of liquid fuels and other chemicals. The reactions taking place in the SMR can be written as follows.CH4+H2O --->CO+3H2  (1)orCH4+2H2O--->CO2+4H2  (2)
Carbon monoxide and hydrogen are produced in the SMR by using steam and methane as the feed. Conventionally, heating processed water in a steam generator produces the required steam, and the methane is usually supplied in the form of compressed natural gas, or by means of a light molecular weight off-gas stream from a chemical or refinery process.
Alternatively, the product gas from an SHR can be used as the feedstock for the SMR by first removing sulfur impurities from the product stream from the SHR with a hot gas cleanup unit that operates at process pressures and is located in between the SHR and SMR. This entire process is described in U.S. patent application Ser. No. 11/489,308, the entirety of which is incorporated herein by reference. However, steam content of up to 50 weight % can be encountered with the producer gas being obtained from the steam-hydrogasification process. This large content of steam can deteriorate the sulfur capture capacity of metal oxide sorbents used in a hot gas cleanup process by shifting the equilibrium of the following reaction toward the backward direction:MO+H2S⇄MS+H2O
where MO and MS denote metal oxide and metal sulfide, respectively. Deterioration of the sulfur capture capacity of the sorbents then leads to (i) higher concentrations of H2S, thereby leading to detrimental affects on the conventional nickel-based catalysts used for steam reforming of methane, catalysts known to be quite vulnerable to sulfur contaminants in an irreversible manner, (ii) greater contamination of synthesis gas with H2S, and (iii) poorer production of synthesis gas due to more frequent process turnaround for catalyst replacement and pressure drop abatement.
Therefore, use of metal oxide sorbents for H2S removal becomes quite stringent as the sorbents are required to function in adverse conditions of large steam content, to the extent sufficient to prevent sulfur-poisoning of the catalyst for steam reforming of methane. Thus, there is a need for an improved process to enhance the operability of hot gas cleanup of steam-hydrogasification producer gas.